This invention relates generally to gas furnace controls and more specifically to the sensing of the presence or absence of a flame in a furnace.
Use of the phenomena of so-called flame rectification to sense the presence or absence of a flame is conventional in gas furnace controls technology. Typically, 120 volt AC power is coupled to a flameprobe through a first capacitor. When there is no flame present a second capacitor coupled to the probe is charged to a selected value, e.g., 5 volts DC, through a resistor connected to a DC voltage source. A change of state device, such as an inverter, has an output connected to a microprocessor and an input connected to the second capacitor. When no flame is present the second capacitor maintains the voltage at the input of the inverter above its threshold so that the output of the inverter is low thereby providing an indication to the microprocessor that there is no flame. When a flame is present, the second capacitor discharges to ground through the flame which acts as a poor diode connected in series with a resistor. When the second capacitor discharges to a level below the threshold, the inverter changes state with its output going high thereby providing an indication to the microprocessor that a flame is present.
A problem exists with this approach however, in view of the low level of current flow. If the inverter or the second capacitor develop too much leakage current to ground an indication of the presence of a flame can occur even at times when, in fact, no flame is present. This can happen because of age, static damage, faulty components or the like. During an ignition sequence when the microprocessor is, in effect, expecting a flame this could result in open gas valves if the circuit failed and indicated the presence of the flame when, in fact, no flame was present. This problem is conventionally dealt with by providing a dual or redundant sensing circuit. In the event that the same signal is not provided by both the primary and the redundant sensing circuit then the logic in the microprocessor maintains the valves in the closed condition. In using the redundant sensing circuit approach an assumption is made that both circuits will not fail at the same time. However, in addition to adding to the cost of the device a problem exists in that if the cause of failure is due to an external event then both circuits could fail simultaneously.